Low-carbon resulfurized free machining steel products are used in small parts, such as screws and nipples, which do not require mechanical properties so high but require good machinability and are manufactured in large quantity by cutting. Free machining steel products containing Pb in addition to S are widely used as free machining steel products having further satisfactory machinability. Pb is a harmful substance which deteriorates health, and demands have been made to reduce the content of Pb in such free machining steel products. Tellurium (Te) is also used in some free machining steel products, but it has toxicity and deteriorates hot workability and must be reduced.
Many investigations have been made to improve the machinability of low-carbon resulfurized free machining steel products, many of which relate to control in number, size and configuration of sulfide inclusions (refer to Patent Documents 1, 2, 3, 4, 5 and 6).
Patent Document 7 pointes that the oxygen content in steel products is important to control the size and configuration of sulfide inclusions. Patent Document 8 indicates that the control of oxygen content in molten steel before tapping is important.
Many techniques relate to control of oxide inclusions (refer to Patent Documents 9, 10, 11, 12 and 13).
The structure and properties (matrix properties) other than such inclusions also significantly affect the machinability, but there are few techniques noting these factors. For example, there are only few techniques such as one specifying a streaky pearlite structure continuously extending in the rolling direction (Patent Document 14) and one specifying the content of dissolved carbon in pro-eutectoid ferrite (Patent Document 15).
Patent Document 16, for example, proposes a low-carbon resulfurized free machining steel product which contains 0.16 to 0.5 percent by weight of S, 0.003 to 0.03 percent by weight of N, and 100 ppm to 300 ppm of oxygen, whose nitrogen (N) is contained in an amount more than conventional free machining steel products manufactured by continuous casting. The resulting free machining steel product can reduce built-up edge formed on a tool face during machining and has machinability equal to or higher than ingot steel products.
Patent Document 1: Japanese Patent No. 1605766 (claims)
Patent Document 2: Japanese Patent No. 1907099 (claims)
Patent Document 3: Japanese Patent No. 2129869 (claims)
Patent Document 4: Japanese Patent Application Laid-open (JP-A) No. 09-157791 (claims)
Patent Document 5: Japanese Patent Application Laid-open (JP-A) No. 11-293391 (claims)
Patent Document 6: Japanese Patent Application Laid-open (JP-A) No. 2003-253390 (claims)
Patent Document 7: Japanese Patent Application Laid-open (JP-A) No. 09-31522 (claims)
Patent Document 8: Japanese Patent Application Laid-open (JP-A) No. 56-105460 (claims)
Patent Document 9: Japanese Patent No. 1605766 (claims)
Patent Document 10: Japanese Patent No. 1907099 (Japanese Patent Application Publication No. 04-54736) (claims)
Patent Document 11: Japanese Patent No. 2922105 (claims)
Patent Document 12: Japanese Patent Application Laid-open (JP-A) No. 09-71838 (claims)
Patent Document 13: Japanese Patent Application Laid-open (JP-A) No. 10-158781 (claims)
Patent Document 14: Japanese Patent No. 2125814 (Japanese Patent Application Publication (JP-B No. 07-11059) (claims)
Patent Document 15: Japanese Patent No. 2740982 (claims)
Patent Document 16: Japanese Patent No. 2129869 (Japanese Patent Application Publication (JP-B) No. 08-949) (claims)